As follows, there has been significant interest in modifying the properties of known or existing materials in a manner which renders the materials suitable for use in environments which normally would adversely affect such materials. For example, one such modifying approach generally relates to coating onto a surface of a substrate material a second material, which has properties which differ from the underlying substrate material.
Various methods exist for coating substrate materials. A first category of coating processes is generally referred to as overlay coatings. Overlay coatings involve, typically, a physical deposition of a coating material onto a substrate. The coating material typically enhances the performance of the substrate by, for example, increasing the erosion resistance, corrosion resistance, high temperature strength, etc., of the substrate material. These overlay coatings typically result in the substrate material having longer life and/or permit the use of the substrate material in a number of environments which normally might adversely affect and/or destroy the utility of the substrate material absent the placement of the overlay coating thereon.
Commonly utilized overlay coating methods include Chemical Vapor Deposition, Hot Spraying, Physical Vapor Deposition, etc. Briefly, Chemical Vapor Deposition utilizes a chemical process which occurs between gaseous compounds when such compounds are heated. Chemical Vapor Deposition will occur so long as the chemical reaction produces a solid material which is the product of the reaction between the gaseous compounds. The Chemical Vapor Deposition process is typically carried out in a reaction chamber into which both a reactive gas and a carrier gas are introduced. A substrate material is placed into contact with the reactant.
The third category of so-called overlay coatings is Physical Vapor Deposition coatings. Physical Vapor Deposition coatings include, for example, Ion Sputtering, Ion Plating, and Thermal Evaporation.
In Ion Sputtering, a vacuum chamber houses a cathode electrode such that the cathode electrode emits atoms and atomic clusters toward a substrate material to result in a sputtered film or coating being deposited on the substrate.
Ion Plating of a substrate material involves the use of a heated metal source which emits metal atoms toward a substrate material which is to be coated. Specifically, an electron beam is typically utilized to excite the metal atoms from the metal source. The excited metal atoms are then directed toward the substrate material to be coated.
Thermal Evaporation also relies on the excitation of atoms from a metal source. Specifically, in a vacuum chamber, a metal source is heated so that metal atoms evaporate from the metal source and are directed toward a substrate material to be coated. The metal atoms then collect as a coating on the substrate.
A second general category of coating formation techniques is known as conversion coating techniques. In conversion coating techniques, a substrate material, typically, is involved in a chemical reaction which modifies the composition and/or microstructure of the surface of the substrate. These conversion coating techniques also can result in desirable surface modification of substrate materials. Typical examples of conversion coating techniques include Pack Cementation and Slurry Cementation.
Pack Cementation and Slurry Cementation utilize diffusion of one or more materials to form a surface coating. Specifically, in each of these processes, a substrate material is contacted with a metal source material such that a metal from the metal source material may diffuse into the substrate material and/or a component of the substrate material may diffuse toward the metal source material. Specifically, for example, in Pack Cementation, a substrate material is buried within a powder mixture which comprises, typically, both a metal which is to react with the substrate material and an inert material. A carrier gas is then induced to flow into the powder mixture so that the carrier gas can carry metal atoms from the metal powder to the surface of the substrate and deposit the metal atoms thereon. Both Pack Cementation and Slurry Cementation typically occur in a retort or vacuum furnace and the carrier gas is free to transport metal atoms from the metal powder to the surface of the substrate material. Typical carrier gases include the halogen gases. Many different approaches to Pack Cementation have been made, however, most of these approaches utilize the above-discussed steps.
Slurry Cementation is quite similar to Pack Cementation, however, in Slurry Cementation, a composition typically is coated onto a surface of a substrate material prior to conducting the diffusion process in a vacuum or retort furnace. In each of Pack Cementation and Slurry Cementation, the temperature of reaction is typically elevated to permit the metal atoms to react with the substrate by solid state diffusion which results in the formation of a coating material.
The above-discussed coating techniques have been briefly addressed herein to give the reader a general understanding of the art. However, it should be understood that many specific variations to the above-discussed techniques exits. Specifically, each of the coating processes discussed above has been discussed in detail in a number of readily available sources, including textbooks, conference proceedings, and patents. For further information relating to the detail of these procedures, the reader is encouraged to consult the literature referred to above. However, even from the brief discussions above, it is clear that each of the techniques suffers from various limitations. For example, in the overlay coating techniques, the physical deposition of a coating onto a substrate material does not insure an acceptable interface between the substrate and the coating. Specifically, because most of the overlay coating techniques simply rely on the use of a physical bonding between the coating and the substrate, the coating may not adhere to the substrate in a desirable manner. Accordingly, the purpose of the coating may be comprised completely. Additionally, all of the overlay coating processes depend on the use of somewhat complex deposition equipment. For example, Chemical Vapor Deposition requires the use of complicated control means for controlling the rate of flow of reactive and carrier gases in a reaction chamber, the ability to handle corrosive alkali gases (e.g., fluorides and chlorides). Accordingly, the equipment utilized for Chemical Vapor Deposition is typically quite expensive.
Moreover, with regard to the so-called conversion coating techniques which are formed by, for example, Pack Cementation and Slurry Cementation techniques, the coatings which result on substrate materials may not be uniform due to the inclusion of solid materials or porosity which result from exposure of the substrate to either or both the powder metal source and/or inert materials utilized in the Pack Cementation or Slurry Cementation processes. Still further, many of the Pack Cementation and Slurry Cementation techniques may require the use of somewhat complex equipment.
The present invention is a significant improvement over all known prior art techniques in that relatively simple equipment can be utilized to achieve a virtually infinite combination of desirable bodies. Specifically, the present invention permits the formation of a coating on substrate materials or the creation of new materials from, for example, solid oxidant precursor materials. The coatings which form are very dense and are substantially uniform in thickness. Additionally, the coatings can be applied in thicknesses heretofore believed difficult, if not impossible, to achieve. Moreover, due to the simplicity of the process and, for example, the rate of conversion of a solid oxidant material to a reaction product, entire solid oxidant bodies can be converted from one composition to another. These and other aspects of the invention will become apparent to those skilled in the art when reading the following sections.